1. Field of the Invention
The present invention relates to a hard magnetic garnet material used for an optical communication system, a Faraday rotator, a method of manufacturing a Faraday rotator and a method of manufacturing a Bismuth-substituted rare earth iron garnet single crystal. The present invention also relates to an optical device using a Faraday rotator and an optical communication system provided with an optical device.
2. Description of the Related Art
Optical communications are currently becoming widespread at an increasing speed in contrast to telecommunications with small transmission capacity. As will be explained later, reasons may be summarized as follows: Optical communication allows, large volume transmission at high-speed, is advantageous in long-distance transmission because it requires fewer relays and is free of influences from electromagnetic noise.
Light is same as a radio wave used for TV or radio broadcasting or wireless communications in the sense that it is an electromagnetic wave. However, frequency of an electromagnetic wave used in optical communications is approximately 200 THz, equivalent to approximately 20000 times that of satellite broadcasting (approximately 10 GHz). Having a high frequency means having a short wavelength and being able to transmit more signals at high speed all the more. By the way, a wavelength (central wavelength) used in optical communications is 1.31 μm (1310 nm) and 1.55 μm (1550 nm).
As is well known, an optical fiber used for optical communications has a double structure of glass pieces of different refractive indices. Since light passing through the central core is repeatedly reflected inside the core, signals are transmitted correctly even if the optical fiber is curved. Moreover, since highly transparent, high purity silica glass is used for the optical fiber, optical communication attenuates only by approximately 0.2 dB per km. Accordingly, optical communications allow transmission of approximately 100 km without amplifiers and allows the number of relays to be reduced compared to telecommunications.
EMI (electromagnetic interference) becomes an issue in telecommunications, while communications using optical fibers are free of noise by electromagnetic induction, which allows information transmission with extremely high quality.
A current optical communication system converts an electric signal to an optical signal using an LD (laser diode) in an optical transmitter, transmits this optical signal through optical fibers and converts the optical signal to an electric signal using a PD (photodiode) in an optical receiver. Thus, elements essential to an optical communication system are LD, PD, optical fibers and optical connectors. Except for a relatively low-speed, short-distance communication system, a high-speed, long-distance communication system also requires, in addition to these device, optical transmission devices such as light amplifier and optical distributor, optical parts applied to these devices such as optical isolator, optical coupler, optical splitter, optical switch, optical modulator, optical attenuator, etc.
What plays a particularly important role in a high-speed, long-distance transmission or a multi-branched optical communication system is an optical isolator. In a current optical communication system, optical isolators are used in LD modules of an optical transmitter and relays. The optical isolator is an optical part that plays a role in transmitting electromagnetic waves only in one direction and blocking electromagnetic waves which are reflected at some midpoint and return. The optical isolator applies a Faraday effect which is a kind of magneto-optical effects. The Faraday effect refers to a phenomenon of rotation of the polarization plane of light which has passed through materials exhibiting a Faraday effect, that is, a Faraday rotator using such as rare earth iron garnet single crystal. The characteristic that the polarization direction of light rotates such as the Faraday effect is called “optical rotary power”. Unlike normal optical rotary power, in the case of the Faraday effect, even if the light propagation direction is reversed, the original condition is not restored, but the polarization direction further rotates. An element using the phenomenon that the polarization direction of light rotates due to the Faraday effect is called a “Faraday rotator”.
The function of the optical isolator will be explained taking an LD module as an example.
An LD is built in an optical transmitter as an LD module integrated with an optical fiber. The optical isolator is placed between the LD and optical fiber and has the function of preventing reflected light from returning to the LD using the Faraday effect. Reflected returning light refers to light which is emitted from the LD, slightly reflected by components such as optical connectors and returned. Reflected returning light causes noise to the LD. The optical isolator that lets light pass in only one direction eliminates this noise and maintains communication quality.
In the case of the LD in the optical transmitter, the vibration direction (polarization direction) of light emitted from the LD is determined to be only one direction, and therefore a polarization-dependent type optical isolator of a simple structure is used. FIG. 6 shows a basic configuration of a conventional polarization-dependent type optical isolator 10. The optical isolator 10 is comprised of a Faraday rotator 11 constructed of a garnet single crystal, a cylindrical permanent magnet 12 that surrounds the Faraday rotator 11 and magnetizes the Faraday rotator 11 and polarizers 13 and 14 that are placed at the front and back surfaces of the Faraday rotator 11. These polarizers 13 and 14 are placed at a relative angle of 45°. With the optical isolator 10, the direction in which light propagates will be called a “forward direction”, while the direction in which light is reflected and returned will be called a “backward direction”.
Then, the mechanism whereby the optical isolator 10 blocks passage of light in the backward direction will be explained. FIG. 7A shows how light in the forward direction passes through the optical isolator 10, while FIG. 7B shows how light in the backward direction is prevented from passing through the optical isolator 10.
As shown in FIG. 7A, linearly polarized light that has passed through the polarizer 13 in the forward direction is rotated 45° by the Faraday rotator 11 and passes through the polarizer 14 placed at a relative angle of 45°. On the other hand, as shown in FIG. 7B, in the backward direction, linearly polarized light that has passed through the polarizer 14 is further rotated 45° by the Faraday rotator 11, and therefore the light cannot pass through the polarizer 13.
The polarization-dependent optical isolator 10 used-for the LD module has been explained above. On the other hand, a polarization-independent optical isolator is also available, such as an optical isolator used for a light amplifier. In the case of a light amplifier, light from an optical fiber enters directly into the optical isolator, and so it is not possible to identify the polarization direction. For this reason, a polarization-independent optical isolator has been developed. The basic configuration thereof is well known and therefore explanations thereof will be omitted here. When the present invention simply refers to an “optical isolator”, it has a concept including both the polarization-dependent and polarization-independent types.
The Faraday rotator affects the performance of the optical isolator. Therefore, the properties of materials composing the Faraday rotator are important factors in attaining a high performance optical isolator. The important factors in selecting materials composing the Faraday rotator include having large Faraday rotation angle with the wavelength used (1.31 μm, 1.55 μm in the case of optical fiber) and having high-level transparency. As a material satisfying these conditions, YIG (yttrium iron garnet: Y3Fe5O12) was used initially, but it was insufficient in terms of mass production and miniaturization.
Then, it was discovered that when a rare earth site of a garnet single crystal was substituted by bismuth (Bi), the Faraday rotary moment was improved drastically, and since then this Bi-substituted rare earth iron garnet single crystal came into use for the Faraday rotator.